In the production of sulfuric acid atomic sulfur is burnt, whereby sulfur dioxide is formed. This sulfur dioxide then is catalytically converted to sulfur trioxide, which by absorption with sulfuric acid itself can be converted into sulfuric acid.
To achieve a yield of sulfur dioxide (SO2) as complete as possible, an atomization of the sulfur as fine as possible and an intermixture with the combustion air as good as possible must be achieved in the burner, in order to achieve a combustion as complete as possible by the shortest route. Suitable burners are described for example in “Winnacker/Küchler. Chemische Technik: Prozesse and Produkte”, edited by Roland Dittmeyer, Wilhelm Keim, Gerhard Kreysa, Alfred Oberholz, Vol. 3, Weinheim, 2005, pp. 37 ff.
To produce an extremely fine distribution of the sulfur, one possibilty consists in blowing the same into the combustion chamber under pressure. Such pressure atomizers also can be designed as binary burners and include a nozzle for the sulfur with a jacket for steam and compressed air to support the atomization. The use of steam has the advantage that the sulfur is maintained at an optimum operating temperature, but at the same time involves the risk that in the case of a leakage water can enter into the system. For a complete combustion of the sulfur, the pressure atomizers (also called “Sulfur Guns”) require a relatively long combustion chamber due to a large combustion flame.
The performance of a nozzle only can be varied in a range from 70 to 100% based on the full load of this nozzle. To be able to operate the plant with different mass flows, it is not possible to feed different mass flows into the individual burner, but rather a plurality of individual burners are connected in parallel. In the case of a partial load operation (weak load operation; below the full load operation) not all burners are used. Another possibility is to provide nozzles of different sizes in a plant, which are exchanged during standstill of the plant. The size of the individual nozzles then is adapted to the respective mass flow.
Furthermore, ultrasonic sulfur burners are used, which are based on the action principle of a gas-operated acoustic oscillator. This oscillator generates a field with high-frequency acoustic waves in a range between 18,000 and 23,000 Hz. When the liquid sulfur passes this field, very small droplets with a diameter between 20 and 160 μm are formed. This process requires sulfur with a feed pressure of about 1 bar above combustion chamber pressure and in addition a very dry gas as propagation medium for the acoustic waves, which must be under a pressure of 2 to 3 bar above combustion chamber pressure. The use of the dry air makes this process very expensive, as about 1,000 Nm3 of dried air cost EUR 120.00 and per ton of sulfur to be converted about 100 Nm3 of air are required.
The rotary atomizer “Luro” is based on a rotating cup into which liquid sulfur is charged. Due to the centrifugal force, a uniform liquid film is formed on the inside of the cup during the rotation. At the cup edge, this liquid film is flung off radially into the combustion chamber and thus is uniformly and very finely distributed, which provides for a very fast evaporation. Due to the fine distribution a short flame of the burner is obtained with a complete combustion, which leads to gases with up to 18 to 19 vol-% SO2. In particular in plants with small capacity gases with about 11.5 vol-% SO2 are employed. The furnace length can be reduced down to 50% of the length required for pressure atomizers and allows an extremely high combustion chamber load of up to 8 GJ M−3. The short, hot flame also leads to lower NOx contents of the waste gas produced. So far, load ranges between 40 and 100% based on the full load range can be run with the Luro burner during ongoing operation.
Especially in times of greatly fluctuating raw material prices, plants often are operated for a short time with distinctly reduced utilization. As the Luro burner is distinctly more complex in its design than a simple pressure atomizer, it cannot simply be replaced by a model which is designed for smaller mass flows.
Furthermore, starting up a plant is facilitated when initially only very small mass flows can be introduced in relation to the full load.